The invention relates to a process for the degreasing, pickling and/or removal of laser scale on ferriferous metal parts, especially by spraying.
The cutting of ferriferous metals by the laser technique is being used increasingly in the sheet fabricating industry. One advantage of the laser cutting technique is its speed, which shortens production times in the sheet-fabricating industry. More and more metal workpieces, especially sheets, are now being cut by laser.
Laser cutting with oxygen as the operating gas produces iron oxide (scale) at the cut edges of the metal. Laser scale is brittle and adheres poorly to the substrate. If sheet metal parts cut to size in this way are lacquered without first removing this scale, the lacquer adheres poorly to these edges, if at all, and easily flakes off. This problem only arises when using the laser cutting technique, since the conventional cutting technique did not normally produce scale at the cut edges of the metal. Before lacquering, such parts only needed to be degreased to ensure adhesion of the lacquer.
Conclusion: The modern and desirable laser cutting technique, in which oxygen has to be used as the operating gas, involves an expensive pretreatment prior to lacquering or powder coating. Examples of suitable pretreatment methods of removing scale are mechanical processes such as sand blasting or dust blasting. However, such mechanical processes are time-consuming and create problems in respect of the dimensional accuracy of the parts.
Another method of removing scale is chemical pickling with acid, where the sheet with scale is dipped in an appropriate bath. This is without problems when using pretreatment plants that work by the dipping process and have an integrated pickling zone. Phosphoric acid pickling baths with ion exchange devices, for example, are known.
The sheet pretreatment is not always carried out by dipping processes. Thus, another pretreatment method that become established along with the dipping process is the spraying process, wherein the parts to be treated are sprayed with the appropriate solutions. The spraying process has the great advantage that the parts can be hung individually on conveyor chains in the appropriate plants. The parts remain hanging on these chains until lacquering has finished. (This is in contrast to the dipping method, where the treatment usually takes place in working baskets and the parts are hung up after the pretreatment and before lacquering.) Processes such as degreasing, degreasing/phosphatizing, phosphatizing [phosphating], etc. are state of the art in the spraying process. The pickling of aluminum in the spraying process is also known and can be controlled by process technology.
However, steel is very rarely pickled and descaled by the spraying process, mainly because the treatment times available in a spraying plant are normally too short for pickling. Another problem is to bring the pickled parts through the rinsing zones to the next treatment stage without them rusting again. After taking up only a few grams of iron, phosphoric acid pickling solutions produce white, sparingly soluble residues of iron (III) phosphate in particular, which can encrust the plant. In the following rinsing bath, as a result of the normal entrainment of product from zone to zone, uncontrollable and usually unwanted iron phosphatizing additionally takes place which, for example, prevents the application of zinc phosphatizing. A spraying pickle based on phosphoric acid is therefore less suitable for the pickling and descaling of ferriferous metals. Loading of the effluent with phosphate is also undesirable.
Sulfuric acid, which is used for pickling aluminum in the spraying process, can hardly be used for steel because of the sensitivity to re-rusting in the rinsing process. This is because the spraying atmosphere is extremely corrosive as it contains not only sulfuric acid but also Fe3+ as a powerful oxidant, so even some special steel alloys can be attacked under certain conditions. Concrete is also fiercely attacked by water containing sulfuric acid. All the parts of the plant should therefore be made of a material that is resistant to these particularly corrosive solutions, which alone incurs a barely justifiable cost. In addition, phosphoric and sulfuric acid salts cause effluent problems and are undesirable. Also, concrete is fiercely attacked by effluents containing sulfate.
Sheets produced by the laser cutting technique, especially when the operating gas for laser processing contains oxygen, cannot be treated with phosphoric acid or sulfuric acid in the spraying process because of the problems described.